1. Field of the Invention
This invention relates to lead-acid cells and batteries and, more particularly, to a cell tray assembly for sealed, maintenance-free, lead-acid cells used in motive power applications.
2. Description of the Prior Art
The advantages that are provided by sealed lead-acid cells and batteries in comparison to conventional, flooded lead-acid batteries are substantial and varied. Sealed lead-acid technology thus offers substantial benefits by eliminating maintenance (e.g., cell watering), expense (e.g., acid purchases), environmental (e.g., expensive waste treatment systems and air-borne acid mist) and safety (e.g., acid burns) concerns.
It is thus not surprising that sealed lead-acid cells and batteries are widely used in commerce today for various applications that have widely differing requirements. In stationary battery applications, the sealed lead-acid batteries provide stand-by power in the event of a power failure. For this type of application, such stationary batteries are maintained at a full state-of-charge and in a ready-to-use condition, typically by floating at a constant preset voltage. Stationary batteries are used for stand-by or operational power in a wide variety of applications, including, by way of illustration, telecommunications, utilities, for emergency lighting in commercial buildings, as stand-by power for cable television systems, and in uninterruptible power supplies.
Uninterruptible power supplies are systems that back-up computers and communications networks. Sealed lead-acid batteries may comprise the power source. The uninterruptible power source allows for the orderly shut-down of computers when there is a sudden interruption in the primary cycle source, such as during a power outage, and provides back-up power for communications networks. The uninterruptible power supply also will accommodate short, or intermittent, losses in power. When there is a power interruption, the batteries in the uninterruptible power system can be subject to rapid discharge.
Numerous stationary power applications require anywhere from 6 to 120 cells or even more. Accordingly, floor space, and sometimes even the height, required for the batteries can present problems. To attempt to minimize the floor space required, it is conventional to position the batteries in various kinds of racks or cabinets. Typically, the battery cabinets that are now being used position the batteries only in a vertical orientation. However, to lessen the floor space required and to accommodate specific applications, sealed lead-acid batteries have been oriented in horizontal positions, as well. Some modular battery stacks and cabinets now available provide a configuration in which the batteries are positioned in the trays in a horizontal position.
Another potential application for sealed lead-acid cells and batteries is a variety of motive power applications in which an array of cells or batteries provides the motive power for vehicles ranging from Class 1 to Class 3 trucks, various automated guided vehicles, mining vehicles and also railroad locomotives.
A variety of design circumstances for motive power applications serves to complicate the situation. Thus, on the one hand, in Class 1 and 2 trucks, i.e., electric rider trucks and electric narrow aisle trucks, respectively, the array of cells or batteries utilized have weight limits that must be met so as to provide the necessary counterbalance for the vehicle. On the other hand, less weight is preferred for Class 3 electric hand trucks since less energy is involved, as long as the appropriate motive power requirements are met.
Also, the physical space available for the motive power source in Class 1 and 2 trucks is often limited. Indeed, the space for the motive power source is often confined so that a maximum space is defined. Still further, in the case of the Class 3 electric hand trucks (often termed "pallet jacks"), there is a height limitation requirement. Accordingly, for safety and practical operational reasons, it is very highly desirable that the operator of the pallet jack be able to see the end of the truck forks. This requirement thus dictates a practical upper limit for the height of the motive power source since the power source is almost always positioned between the operator and the forks.
The performance requirements for motive-powered vehicles are quite different from the performance requirements for stationary battery power sources. In stationary applications, the depth of discharge in service is relatively shallow, and the number of discharges is small, as most batteries are in float service. In direct contrast, motive power applications require relatively deep depths of discharge to be achieved on a continuous cycling basis over a period of time. Indeed, a common requirement for Class 1-3 trucks is that, in an 8-hour shift, the cell or battery assembly must be capable of delivering an 80% depth of discharge and that performance is required for about 300 cycles per year with a useful service life under those conditions of 4 or 5 years.
Using sealed lead-acid cells and batteries in motive power applications would offer substantial advantages over the use of flooded lead-acid batteries. Sealed lead-acid cells and batteries thus allow opportunity charging (i.e., charging for short periods of time when the vehicle is not in service) while such charging could not be satisfactorily used with flooded lead-acid batteries. Sealed lead-acid cells and batteries typically offer a more durable product, due to less shedding of active material and for other reasons. Sealed lead-acid cells and batteries offer improved performance for cold storage applications and the like in comparison to that of flooded lead-acid batteries. Cell watering, periodically needed in the maintenance of flooded lead-acid batteries, is difficult to carry out in some applications because the battery is relatively inaccessible. Indeed, the overall expenses for maintaining flooded lead-acid batteries for their useful service life can be substantial.
Yet, despite the many advantages that would be derived by using sealed lead-acid cells in comparison to other lead-acid battery types, sealed lead-acid cells have only made slight penetration into this market because of the inability to match the performance characteristics of conventional, flooded batteries for such motive power applications. Lead-acid cells and batteries in which the electrolyte is present in gelled form are also promoted for this type of application, but this type of battery likewise has made little penetration due to unsatisfactory performance.
There accordingly exists a need to provide an assembly of sealed lead-acid cells that retains the advantages of using that type of lead-acid cell, yet which can satisfy the performance requirements of motive power applications.
It is accordingly a principal object of the present invention to provide a sealed lead-acid cell tray assembly that provides superior performance for powering a vehicle requiring relatively deep depths of discharge on a repetitive cycling regime.
It is a further object of this invention to provide a sealed lead-acid cell tray assembly which allows ease in manufacture, assembly and installation as well as adaptability to a wide variety of motive power applications. A more specific object provides a cell tray assembly that may be utilized in spark-proof and explosion-proof designs.
A further object lies in the provision of a sealed lead-acid cell tray assembly which allows, when desired, the ability to maximize the motive power performance from given space requirements in comparison to other sealed lead-acid configurations.
Yet another object of the present invention provides a cell tray assembly which facilitates the dissipation of the heat generated during service.
Another object of the present invention is to provide a cell tray assembly that may be used in existing motive power applications without the need to modify the area allotted for the motive power source, i.e., the cell tray assembly of this invention may be used interchangeably with existing motive power sources.
A still further object of this invention provides a sealed lead-acid tray assembly which minimizes the opportunity for accidental shorting and the like.
These and other objects and advantages of the present invention will be apparent from the following description and drawings.